The demand for hydrogen for use in refining operations increases rapidly, and this has become a matter of acute concern. Shifts to higher sulfur level feeds as the demand for distillate products rise and environmental requirements (which restrict the sulfur content of refined products) are a major cause of the increasing demand. Thus, e.g., hydrotreating, hydrodesulfurization and hydrocracking operations consume more and more hydrogen. On top of this commercialization of new synthetic crude development processes will further increase this demand. More efficient methods for the recovery of hydrogen from off-gas streams are now mandatory.
The use of solid adsorbents for selectively separating hydrocarbons such as methane, ethane, and the like, and other non-polar and polar compounds such as nitrogen, carbon monoxide, carbon dioxide, and the like, from hydrogen has been disclosed in various publications, inclusive of both the technical and patent literature. Adsorbents disclosed in the literature for such purpose include generally, activated charcoal, activated bauxite, activated alumina, silica gel, silica alumina, molecular sieves, inclusive of synthetic zeolites and the so-called carbon molecular sieves, and the like. A complete cycle of operation in a selective adsorption process generally requires regeneration of the exhausted solids adsorbent, and to provide a continuous operation, the adsorption unit must be cycled between adsorption and desorption modes; preferably, both adsorption and desorption being conducted concurrently. The adsorbed material can be removed from the adsorbent solids by purging the solids with an inert gas, or by displacement of the adsorbed material from the adsorbent solids by contact with a more strongly adsorbent compound, without change of temperature or pressure. However, a thermal or pressure swing technique is generally used to regenerate the adsorbent. In thermal swing processes, a bed of the adsorbent solids is heated to a temperature above that at which the bed of adsorbent was employed in the adsorption mode, sufficient to reduce the adsorptive tendency of the adsorbate which can then be removed by a stream of purge gas. In pressure swing processes, the ambient pressure upon the adsorbent solids is dropped below that at which the bed of adsorbent was employed in the adsorption mode, sufficient to reduce the adsorptive capacity of the adsorbate which can then be removed by a stream of purge gas.
In most selective adsorption processes, a fluid, notably a gas or vapor, is brought into contact with a bed, or beds, of the adsorbent solids, one or a plurality of beds being contacted with the gas or vapor to effect a selective separation of a compound, or compounds, from the gas in an adsorption step, while the adsorbed compound, or compounds, is being desorbed in one or a plurality of other beds to regenerate the adsorbent solids, supra. Whereas fixed beds of adsorbent are the most common, fluidized beds have been used to reduce certain of the recognized deficiencies of fixed bed operations, viz., extensive valving and manifolding, and heat waste due to cyclically heating and cooling flow lines, vessel walls and internal vessel components. Nonetheless, fluidized bed adsorption processes too have their limitations, albeit they eliminate or reduce the need for valving, manifolding, as well as facilitate the transport of solids, and provide good heat transfer characteristics, due largely to solids circulation and solids back-mixing. In fact, this latter feature, the very feature which assures good heat transfer and isothermal conditions within a fluidized bed in itself proves disadvantageous in adsorption processes. Consequently, to lessen solids back-mixing a plurality of vertically spaced shallow fluidized beds are generally employed to provide stagewise contacting between gas and adsorbent. The complexity introduced into fluidized adsorbent processes due to such staging has militated against the more general use of fluidized beds for selective separations in gas treating.
In U.S. Pat. No. 4,283,204, there is disclosed a process for the separation of contaminants from gases using a magnetically stabilized fluidized bed as disclosed in U.S. Pat. No. 4,115,927, issued Sept. 26, 1978, to R. E. Rosensweig. Adsorption and desorption of contaminants from the feed takes place in a fluidized (i.e., expanded and levitated) bed accomplished without the need for a plurality of vertically spaced shallow beds by employing an applied magnetic field to stabilize or structure the fluidized bed. A bed, so stabilized, takes on the appearance and many of the characteristics of a fixed bed with substantially no gross solids circulation or recirculation (except for the plug flow movement of the solids through the vessels) and there is very little, if any, gas by-passing. The application of the magnetic field enables the application of superficial fluid flow rates 2, 5, 10, or 20 or more times the superficial fluid flow rate of the fluidized bed at incipient fluidization in the absence of the applied magnetic field, concomitant with the absence of bubbles. As the superficial fluid velocity is increased, the pressure drop through the bed is similar to that which would be expected from a normal fluidized bed without the application of a magnetic field; it increases to the bed weight support value at the minimum fluidization velocity, and then remains relatively constant as the fluid velocity is increased. This stably fluidized bed condition persists even as the solids are continuously moved in a descending, substantially plug flow manner through the contacting vessels. Countercurrent staged flow of the solids with respect to the flow of gases used to fluidize the bed is achieved, this resulting in reduced overall investment and operating cost, as contrasted with prior art processes. Moreover, better retention of the particles within the bed directly resulting from the influence of the applied magnetic field makes possible the use of smaller particles, this providing better heat and mass transfer between the reactants.
Whereas however, this proves an admirable process for the separation of contaminants from gases, further improvements are nonetheless desirable, and necessary.
It is, accordingly, the primary objective of this invention to provide a new and improved process of this general type, particularly one suitable for the more efficient purification of a vapor, or gas feed.
A further object is to provide a process utilizing the application of a magnetic field for the recovery, and purification of hydrogen by the selective adsorptive separation of non-hydrogen components from off-gas streams which contain hydrogen.
A yet further, and more particular object is to provide a magnetic bed process, as characterized, for the selective adsorptive separation of a hydrocarbon component, or hydrocarbon components, from an admixture which contains hydrogen in admixture with a hydrocarbon, admixture of hydrocarbons, especially normally gaseous hydrocarbons, or these alone or contaminated with non-hydrocarbon compounds (i.e., acidic, polar or non-polar compounds), to provide hydrogen at a selected level of purity with higher hydrogen recovery than heretofore believed feasible.
A more specific object is to provide a magnetically stabilized bed, temperature and partial pressure-swing process for hydrogen recovery from process streams.
These objects and others are achieved in accordance with the present invention, which comprises an improved adsorptive process for the more efficient recovery of hydrogen from a hydrogen-containing feed wherein particulate adsorbent solids are provided with a magnetizable component and circulated between an adsorption zone for the selective adsorption of non-hydrogen components which concentrate the hydrogen within the off gas, and a desorption zone wherein the adsorbed components of the particulate adsorbent solids are desorbed in a heat swing operation, or heat/partial pressure swing operation, the said particulate adsorbent solids regenerated, cooled, and recycled to said adsorption zone, especially improvements in the adsorption/hydrocarbon displacement operation which is characterized by (a) contacting countercurrently said feed, preferably adiabatically, in said adsorption zone with a downwardly moving, fluidized bed of the particulate adsorbent solids, magnetically stabilizing said bed to suppress the gross circulation of adsorbent solids within said bed, while absorbing hydrocarbons from said feed on said bed, withdrawing from said adsorption zone a hydrocarbon-denuded gas stream of higher hydrogen content than that contained in the feed entering said magnetically stabilized adsorption zone, withdrawing hydrocarbon enriched, hydrogen-containing particulate adsorbent solids from said magnetically stabilized adsorption zone and passing same to a hydrogen displacement zone, and then (b) contacting countercurrently said hydrocarbon enriched, hydrogen containing particulate adsorbent solids in said hydrogen displacement zone by passing same in plug flow and contacting same with a hydrocarbon gas sufficient to substantially displace hydrogen, passing the displaced hydrogen into the adsorption zone, and removing the hydrogen-denuded, hydrocarbon-containing particulate adsorbent solids from said hydrogen displacement zone and transporting same to the desorption zone.
In said magnetically stabilized bed adsorption step (a), supra, the feed is injected into the bottom of the zone and flows countercurrent to the slowly descending stably fluidized bed of adsorbent, the gas rising within this zone sufficient to overcome gravity and cause bed expansion and fluidization (the magnetic field being applied in the same direction as the field due to gravity), increasing in its hydrogen content as it rises to the top of the bed; and conversely, the hydrocarbon content of the adsorbent solids increasing as the solids descend to the exit side of the bed. Preferably, the particulate adsorbent solids enter the adsorption zone at a temperature ranging from about 40.degree. F. to about 300.degree. F., preferably from about 130.degree. F. to about 250.degree. F., and exit the adsorption about 10.degree. F. to about 150.degree. F. higher, preferably about 10.degree. F. to about 100.degree. F. higher, than the temperature of the entering particulate adsorbent solids, since adsorption is an exothermic reaction dependent to some extent upon the nature of the adsorbent, and adsorbent solids. In a typical operation, wherein preferred adsorbent solids were employed, viz., a ferromagnetic 5A molecular sieve composite and a ferromagnetic activated carbon composite, the particulate adsorbent solids entered the zone at a temperature of about 200.degree. F. with the temperature rising to about 260.degree. F. at the gas inlet point due to the exothermic heat of reaction.
In hydrocarbon displacement, step (b), supra, the bed of hydrocarbon enriched particulate adsorbent solids removed from said adsorption zone is contacted with a stream of hydrocarbon gas, suitably a recycle hydrocarbon gas stream from the process, the hydrocarbon gas stream entering the bottom of this zone to displace hydrogen from the interstices and pores of the spent particulate adsorbent solids, the volume of hydrocarbon gas added being essentially equal to the volume of gas displaced, viz. the sum-total of hydrogen plus the residual feed gas components contained within the interstices and pores of the particles. In displacing hydrogen in this manner, virtually no hydrogen leaves the zone with the adsorbent solids, and an interface is formed below which virtually no hydrogen is found. The particulate adsorbent solids removed from the zone are substantially free of hydrogen, this increasing overall hydrogen recovery.
When the adsorbent solids are introduced into the hydrogen displacement zone as a slumped bed, moving in plug flow and countercurrently contacted with the hydrocarbon displacement gas, the upper portion of the hydrogen displacement zone is operated at the temperature at which the countercurrently contacted, plug flow solids enter the zone from the adsorption zone, the exit temperature increasing generally from about 10.degree. F. to about 100.degree. F., generally about 20.degree. F. to about 50.degree. F. With said preferred ferromagnetic 5A molecular sieve and ferromagnetic activated carbon the exit temperature typically ranges about 30.degree. F. above the entering temperature of the solids. Suitably, and preferably, a temperature gradient can be provided by the addition of heat; the bottom of the bed being operated at a temperature ranging from about 60.degree. F. to about 450.degree. F., preferably from about 160.degree. F. to about 350.degree. F. above the top bed temperature.
In another of its preferred aspects, the hydrogen displacement zone is operated by the imposition of a magnetic field upon a downwardly flowing fluidized bed of the particulate adsorbent solids, sufficient to stabilize the bed; and the bed countercurrently contacted with the hydrocarbon gas. The temperature of the bed at the top ranges from about 50.degree. F. to about 400.degree. F., preferably from about 150.degree. F. to about 300.degree. F., and the bottom of the bed is operated at a temperature of about 60.degree. F. to about 450.degree. F., preferably from about 160.degree. F. to about 350.degree. F., above the top bed temperature. The temperature gradient maintained on the bed displaces essentially all traces of hydrogen from the particulate adsorbent solids which exit from the bed.
The desorption, or regeneration phase of the operation is begun by transporting the hydrogen denuded, hydrocarbon-containing particulate adsorbent solids to the desorption zone. (c) In said desorption zone, a fluidized bed of said hydrogen denuded, hydrocarbon-containing particulate solids is formed and heated at temperature sufficient to desorb hydrocarbons from said particulate solids, hydrocarbons are removed from said zone, and the particulate adsorbent solids which contain at least residual amounts of hydrocarbons are passed to a stripping zone, and the hydrocarbons stripped therefrom as by contact with a stripping agent, suitably hydrogen. Preferably, however, the hydrocarbons are stripped from the adsorbent solids by stripping with steam. (d) In said steam stripping zone, the residual hydrocarbons-containing particulate adsorbent solids can be formed as a moving, slumped bed, but preferably are formed as a fluidized bed, magnetically stabilized to suppress the gross circulation of adsorbent solids within the bed, while said bed is contacted countercurrently with steam to desorb, and substantially displace the residual hydrocarbons.
Desorption of the hydrocarbons from the hydrogen-denuded, hydrocarbon-containing particulate adsorbent solids within the desorption zone (c), supra, is conducted in a fluidized bed which is heated to desorption temperature, viz., temperatures ranging from about 300.degree. F. to about 600.degree. F., preferably from about 350.degree. F. to about 450.degree. F. The heat is generally supplied by means of a heat exchanger, e.g., a coil mounted within the fluidized bed of solids through which a pressurized hot fluid, usually steam, is passed. The partially desorbed solids are passed from the desorption zone to the hydrocarbon stripping zone (d) into which steam is directly injected to maintain desorption temperature, the steam simultaneously stripping hydrocarbon via competitive adsorption and partial pressure reduction, and heating the adsorbent solids, at least in part via water adsorption. The temperature of the desorption zone generally ranges from about 300.degree. F. to about 600.degree. F., preferably from about 350.degree. F. to about 450.degree. F. The wet particulate solids are then transported to a steam displacement, or water removal zone, preferably involving a sequence of steps wherein hydrogen is used first to displace steam from the interstices and pores of the solids, and then water is stripped from the sorbent solids while effecting evaporative cooling in a fluid bed cooling zone prior to recycle of the adsorbent particles to the adsorption zone.
A sequence of stages is preferably employed in the process of removing water, and cooling the adsorbent particles. In the first such stage product hydrogen is used to displace the steam from the interstices and pores of the particulate adsorbent particles. The hydrogen countercurrently contacts the adsorbent particles at a rate to essentially equal the volume of steam, or water, in the interstices and pores. In displacing steam in this manner virtually no steam or gaseous hydrocarbon is carried to the water removal stage wherein the water adsorbed on the adsorbent particles is removed. The steam displacement stage minimizes the amount of stripping steam required in the hydrocarbon stripping zone (d) by more effectively utilizing the steam injected into the zone.
Additionally the steam displacement stage prevents any residual gaseous hydrocarbon left in the interstices or pores of the adsorbent particles from contaminating the purified hydrogen which is recovered downstream in a more advanced stage of the operation.
In the second stage of the water removal/cooling zone, i.e., the evaporative stage cooling stage, the adsorbent particles are countercurrently contacted with product hydrogen which has been injected into the bottom of the stage at a rate sufficient to strip essentially all the water from the adsorbent particles. The removal of water evaporatively cools the adsorbent particles from their inlet temperature ranging from about 300.degree. F. to about 550.degree. F., preferably from about 350.degree. F. to about 450.degree. F., to an exit temperature ranging from about 200.degree. F. to about 500.degree. F., preferably from about 250.degree. F. to about 350.degree. F. The dried adsorbent solids are then transported to a cooling zone for adjustment of the temperature of the solids to adsorption temperature.
The cooling zone is one wherein a fluidized bed of regenerated particulate adsorbent solids is established, and maintained in heat exchange relationship with a coolant, suitably cooling water with heat rejection to the cooling water. Within the fluid bed cooling zone the temperature of the adsorbent solids is adjusted to the desired adsorption temperature, and then transported to the adsorber.
In a preferred mode of operation these several zones are employed to provide a continuous single train temperature swing adsorption/desorption process utilizing countercurrent plug flow of sorbent and gas, which permits the use of small particle size sorbent to provide faster sorption-desorption rates, and fluid bed heat transfer zones providing optimum staging and approach to equilibrium. A specific process of this type providing high recovery (e.g., 95+%) of high purity (e.g., 95+%) hydrogen utilizes an adiabatic, magnetically stabilized adsorption section for countercurrent contacting, with substantially complete displacement of the feed gas from the adsorbent solids; a staged desorption section, especially a two-zone desorption section, inclusive of a first desorption stage which uses a fluidized bed to heat the adsorbent solids to desorption temperature, and a second desorption zone which utilizes steam for stripping and to provide adsorption heat; and a staged water removal/cooling section, especially a three-step cooling section, a first wherein hydrogen is used to displace steam and residual gaseous hydrocarbons from the interstices and pores of the particulate adsorbent solids, a second wherein water is removed from the adsorbent solids to provide evaporative cooling, and a third wherein further cooling to adsorption temperature is provided in a fluidized bed. The separation takes place at feed gas pressures and can be achieved at pressures ranging from about 50 pounds per square inch gauge (psig) to about 1200 psig, preferably from about 200 psig to about 700 psig, fuel gas being returned at essentially feed gas pressure. High purity and recovery are obtained by using a countercurrent flow of feed gas.
These features and others will be better understood by reference to the following more detailed description of the invention, especially a preferred embodiment shown by reference to the attached drawing to which reference is made.